The number of people registering to vote has increased markedly, adding a new generation of voters to the electorate and making the outcome of the election even more volatile, according to a survey of marginal constituencies conducted by the Guardian.

Figures collected from more than 20 of the most marginal areas following Tuesday's deadline to register to vote indicate across-the-board increases in the electorate, compared with the last general election, suggesting that turnout could soar on 6 May.

In one area, the increase is as high as 17%, and there are also indications of a dramatic surge in people requesting postal votes.

The volume of postal vote requests doubled in some areas compared with 2005, putting electoral administration systems under pressure and raising new concerns about postal voting fraud and the extent to which parties are manipulating the system to boost their vote.

Candidates in three separate areas told the Guardian that postal votes were central to their campaign. One said they had redoubled their efforts to sign people up for postal votes as the margins between the parties tightened and the prospect of a hung parliament and a second election this year grew ever more likely.

The Liberal Democrats could benefit disproportionately from the boost in numbers and postal votes. The Electoral Commission has said that those expressing an interest in registering were disproportionately in the youngest age bracket of 18-24 – the same group that recent polls show backing the Lib Dems. Previous trends show that people who use a postal vote send it quickly after they receive it meaning that millions of people will be voting in the next week when the party is still high in the polls on the back of the first TV debate.

The Guardian approached 25 local authorities to request figures on voter registration and postal votes after Tuesday's deadline for applications. Nearly every response revealed an increase in both figures compared with the 2005 election after being adjusted for boundary changes.

A spokesman for Wandsworth council, which covers three constituencies, including the marginal seat of Battersea, described a "terrific surge" in applications. There had been predictions that interest in voting would fall following the expenses saga. But today's figures suggest that reforms to the registration system and possibly the tight nature of this election could be countering that effect.

In Sittingbourne and Sheppey the electorate increased by 17% after adjustments for boundary changes and postal votes went from 6,429 in 2005 to 9,529 this week. In Crawley, registrations have gone up by 1.2%, but postal votes by 11% to 12,076. Ceredigion, which had a very minor boundary change, saw an 11% increase in registrations to 59,607 and postal votes more than double from 2,954 to 6,732.

Registrations were all but unchanged in Edinburgh South since 2005, but postal votes had increased by 60%. Brian Brown, head of electoral administration for Edinburgh South, said they had to extend deadlines to send postal votes out in order to cope with the surplus. "We've had a significant surge in late applications," he said.

Barnet council, which covers three constituencies, including the marginal Finchley & Golders Green, reported 12% more registrations and 63% more postal votes. Brighton council, which also covers three constituencies, had a 2% increase in registrations compared with the last election and 40% increase in postal votes.

Paul Holloway, head of electoral services Brighton and Hove city council, said: "We have seen a healthy surge in the numbers of residents making contact to ensure they are registered to vote... we have averaged around 2,000 alterations to the register every month and the post we receive recently has tripled.

"Our campaigns have also targeted students and just yesterday a groupof university students came into the office with a whole box load ofregistration forms."

Some 500,000 registration forms were downloaded from the Electoral Commission website and 50,000 calls were made to a registration helpline. A survey of visitors to the website found that 40% were aged 18-24.

Jenny Watson, the chair of the Electoral Commission, said that increases in registrations followed major reforms of the system which allowed people to register in the first 11 days of the campaign whereas previously, people could only register up until the election was called. A new online system that allows people to download application forms has also made the system more user-friendly, she said.

Three candidates, whose constituencies were not covered in the survey, described how postal votes were central to their canvassing. One Labour candidate said: "Normally at this point we would consolidate and go back to core voters. But this time we are doing even more. We've got huge numbers of postal votes signed up – probably another 4,000.

"If they are voting Labour we want to know: 'Can you get there? Are you on holiday? How can we help?' We're going round the care homes because they are going to need a postal vote and we need them."

It is not illegal to encourage supporters to register for a vote, but political activists are not allowed to touch or handle voters' ballot papers. There is a national code of conduct to which the parties have signed up to prevent fraud, but some areas are supplementing that with even tougher guidelines.

In Kettering this year, one in four registered voters have requested a postal vote. David Cook, the chief executive of Kettering council and the acting returning officer, said: "Some years ago, highly publicised cases of fraud in other areas damaged confidence in postal voting. All the local party candidates, their respective agents and I came together to agree a local code to help further assure voters that the postal voting system was both convenient and safe."